Jeff Passan (a favorite) has a very good breakdown of the big trade that just sent James Shields to the Royals and uber prospect Wil Myers to the Rays.
This trade very much reminds me of the trade that send Josh Beckett to the Red Sox and uber prospect Hanley Ramirez to the Marlins. That trade ended up being a win for both teams. The Red Sox got a final piece that allowed them to meet their goal of winning the World Series while the Marlins got a young superstar under team control. The extra piece that ended up giving value way beyond what was expected in that trade was Mike Lowell who would go on to become the World Series MVP for Boston. Wade Davis is giving off that vibe for me in this trade for Kansas City. Please note that I'm not saying Shields is the final piece for a Royals World Series - their goal is simply to be competitive in the AL Central and I think this trade helps them with that goal.
It should also be noted that if sabermetric GM Theo Epstein was in place at the time for Boston then that trade may never have happened and the Red Sox could still be searching for their second Series win. Point is sometimes sabermetrics doesn't tell the whole story. I consider myself a stats guy but I also realize that games aren't won on the spreadsheet.